Voice-controlled two-way telephone set



J. N. WALTER VOICE-CONTROLLED TWO-WAY TELEPHONE SET Filed NOV. 29, 19472 Sheets-Shes?. l

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IN1/Europ Jl N.. WALTER By l ATTORNEY Patented Jan. 31, 1950VOICE-CONTROLLED TWO-WAY TELEPHONE SET James N. Walter, East Elmhurst,N. Y., assgnor to Bell Telephone Laboratories, Incorporated, New York,N. Y., a corporation of New York Application November 29, 1947, SerialNo. '788,856

'7 Claims. 1

The invention relates to two-way signal transmission systems, andparticularly to circuits for controlling the direction and volume ofsignals transmitted over such systems.

The invention is broadly applicable to a twoway signal transmissionsystem including, at least at its terminals, separate one-way amplifyingpaths for the signals transmitted in opposite directions, withassociated signal-controlled switching circuits for controlling the lossvalues of variable attenuation networks or other loss devices in the twoamplifying paths so as to change the direction and control the volume ofthe transmitted signals. It is especially applicable to, although notlimited to, a voice-controlled two-way telephone set, for example, avoice-controlled operators two-way telephone set, comprising separatevoice signal transmitting and receiving paths each including waveamplifying means, connected to two-way line terminals, the transmittingpath normally including a high loss and the receiving path low loss sothat the set is normally conditioned for signal receiving only, and acontrol circuit connected to the input of the transmitting path in frontof the normally high loss device therein, which is operativelyresponsive to the applied voice currents of the operator, when shetalks,to insert a high loss in the receiving path and remove the normal highloss from the transmitting path, so as to allow the operator to talkwith a distant party over the as scciated two-way line while preventingexcessive reception in the operators receiver.

An object of the invention is to improve the operation of such two-waysignal transmission systems or telephone sets.

A more specic object is to reduce objectionable transient disturbancesin the signal transmission paths of such systems or sets caused by theswitching of signal transmission from one direction to the other.

These objects are attained in accordance with the invention mainly bythe provision of means associated with the signal-controlled switchingcircuit for making part of it relatively fast operating when switchingfrom the transmitting to the receiving condition and relatively slowreleasing when performing the reverse switching operation.

The various objects and features of the invention are explained in moredetail in the following complete description to be read in conjunctionwith the accompanying drawings, in which:

Fig. 1 is a functional schematic of -a portion of a two-way telephonesystem, of the type to which the invention is applicable;

Figs. 2 and 3 show curves respectively illustrating the sequence ofswitching operations in the system of Fig. l which would result inobjectionable switching transients and a sequence of switchingoperations which will reduce such switching transients; and

Fig, 4 shows schematically a known type of voice-operated operatorstelephone set modified in accordance with the invention.

The two-way telephone `circuit s-hown in the functional single lineschematic of Fig. 1 comprises a voice signal transmitting circuit TCincluding the telephone transmitter T and the transmitting amplifier TA,and a voice signal receiving circuit RC including the telephone receiverR and the receiving amplier RA, connected through the three-windingtransformer TI to the two-way line L. A vario-losser or variable losspad VL( is inserted in the transmitting circuit TC in front of theamplifier TA, and a similar vario-losser or variable loss pad VL2 isinserted in the receiving circuit RC in front of the amplifier RA. Thecommon control circuit CL for the vario-lossers or variable loss padsVLI and VL2 has its input connected across the transmitting circuit TCin front of the variolosser pad VLl. The control circuit CL includes acontrol amplifier CA in its input and two separate detector circuits DIand D having their inputs connected to the output of the controlamplifier CA and their outputs respectively arranged to control thevario-losser pad VLI in the transmitting circuit TC and the vario-losserpad VLZ in the receiving circuit RC.

The vario-losser pad VLI is normally biased or adjusted so that, in theabsence of signal transmission in the transmitting circuit TC, it willprovide its `maximum loss, say, 25 decibels, in the latter circuit, andthe vario-losser pad VLZ is normally biased or adjusted so that, in

, the absence of signal transmission in the transmtting circuit TC, itwill provide its minimum loss, say zero or approximately zero decibels,in the receiving circuit RC. With the telephone circuit in this normalcondition, the normal high loss of the vario-losser pad VLI willattenuate the room noise impressed on the input of the transmttingcircuit so that the amp-lined portion thereof diverted into thereceiving circuit will not be of sufcient amplitude to mask signalsreceived by that circuit from the line L. The voice signals of a distantparty incoming over the two-Way line L and impressed by transformer TIon the receiving circuit RC, while the operators set is in thiscondition, will pass through the vario-losser pad VL? with littleattenuation and after amplication in the receiving amplifier RA will beheard in the operators receiver R. A portion of the incoming voicesignals reproduced by receiver R will be fed back as sidetone from there ceiver Rto the transmitter T, and willbe impressed by the latter onthe transmitting circuit TC. This sidetone energy, however, largelydissipated in the vario-losser pad VL! in front of the transmittingampliiier TA, because of the normal high loss condition of the former,so that the amount of the sidetone ,passing through the transformer Tiinto the receiving cir cuit RC will be insucient to appreciably distortthe received voice signals.

If, however, the operator talks into her transmitter T, a portion of theresulting voice currents in the output thereof will be diverted into the.control circuit CL in which it will be amplied by the co-ntrol amplierCA in accordance with its gainl'setting. The resulting amplied voiceVcurrents will be divided by the transformer T2 between thedetectorsDland Dand will be detected thereby. The detected voice currents in theoutput of thedetector circuit Dl will be applied to the vario-losser padVL in transmitting circuit TC in such manner as to bias or causeadjustment ofthat vario-losser pad to its mini- Y mum l-oss condition,say, zero or nearly Zero decibols, and the. detected voice currents inthe output of the detector circuit D will be applied to thevario-losserpadVL2 inthe receiving circuit :RC in such manner kas tobias orladjustthe latm tors .outgoing voice currents ypassing throughtransformer Ti tothe receiving circuit RC will be suiiiciently attenuated bythenow highloss of will be Y the vario-losser pad VLZin that circuit so as toprevent, whiiethe `operator continues to talk, excessive reception-innerreceiver R. For the assumed losses of the circuits TC and RC includingthe vario-losser pads VL! and VL2, and

the selected gains ofiamplifiers TA and RA, a

net attenuation of l5 decibels exists between the transmitter T and thereceiver R. When the operator ceases talking `so as to reduce thecurrents in the outputs of the detectors DI and D2 in the control.circuit CL to zero, the vario-losser pad'VLl in the transmitting circuitTC `will restore to its normal high loss condition and the vario-losserpad VL2 in the receiving circuit RC will restore toits normal low losscondition to maintainsanet sidetone attenuationof decibels.

The sequence of vario-losserpad operations or restorations whenswitching the set from the transmitting to the receiving condition orvice versa, is extremely important as shown by the appropriately labeledloss-time and gain-time curves of Figs. 2 and 3. If, in going from thetransmitting to the receiving condition, the high loss decibels) is`removed `from the receiving circuit RC before the high loss (25decibels) is reinserted in the transmitting circuit TC, as indicated bythe righthand curves of Fig..2 a mothe receiving to the transmittingcondition, the

high loss (25 decibels) is removed from the transmitting circuit TCbefore the high ioss (25 decibels) is inserted in the receiving circuitRC, as 'indicated by the left-hand curves of Fig. 2 a similar momentarynet circuit gain of 10 decibels existsin thesidetone path. As a result,during the switching operation in either direction, the receiver Rproduces a momentary loud burst of sidetone which is very unpleasant tothe operator. These switching transients may be substantially removed byadjusting the time constants of the signal-controlled switching circuitCL so that the vario-lossers VLI and VLZ 4are operated in the sequenceindicated by the curves of Fig. 3, in which, relative to the transmitvario-losserVLl, the receiving vario-losser VL2 is fast operate and slowrelease. .With the sequence of operations, the sidetone is momentarilyattenuated to an abnormally high degree to produce the brief slots S,which are scarcely noticeable, much less objectionable.

One arrangement of the invention for attaining the desired timeconstants for vario-losser operation and release in accordance with theinvention vso as to prevent objectionable transient disturbances in thesignal transmission paths when switching between the transmitting 'andreceiving conditions, is shown in Fig. 4 in connection with avoice-controlled operators twomway telephone set similar to thatdisclosed in the United States Patent No. 2,332,430, issued October 19,1943, to U. S. Berger.

In the voice-controlled operators telephone set similar` to that shownin the aforementioned Berger patent, shown inFig. 4, only those partsnecessary to explain the operation of the arrangement of theinvention'have been illustrated and described in detail, they otherportions of the Berger arrangement being either illustrated byappropriately labeled boxes or eliminated entirely in order to simplifythe description.

The operators telephone circuit of Fig. 4 includes the voicetransmitting branch TC and the voice receiving branch RC, coupled by thethreewinding transformer Tl to a two-wire terminating rsection TS whichis adapted for connection in any suitable manner to a selected telephoneline.

The voice transmitting branch includes in order reading from leittoright, telephone transmitter T and associated energizing battery BI, thetransmitting.vario-losser pad TVL and the transmitting amp-liner TAhaving its output connected to one winding l of thetransformer Tl. Thevoice receiving branch R-C which has its input connected kto a secondwinding 2 of ktransformer Ti includes the frequencycharacteristic-correcting networks Ni in itsinput followed by thereceiving vario-losser pad RVL, the receiving amplifier RA and thetelephone receiver R.

The transmitting vario-losser pad TVL comprises a short section of linecoupled by the transformers T2 and T3 between the transmitter T and theamplifier TA with a network comprising a linear resistor 3 and twonon-linear elements, each of which may comprise a copper-oxide typerectifier, y4 and 5 in series, shunted by a linear resistor 6, connectedin series with each side of the line section, and the linear resistors Iand 8 shunted across the line section at the points indicated. Thereceiving vario-losser pad RVL comprises a bridge having a copper-oxiderectifier poled as indicated in each of its four arms, and one diagonal(horizontal) connected in series with the linear resistors 9 and Illacross the receiving branch RC in front of the receiving amplier RA.

The control circuit CL for the vario-losser pads TVL and RVL has itsinput connected across the secondary winding of transformer T2 in thevoice transmitting branch TC. It includes in its input the delayedoperate vario-losser pad HVL followed by an alternating currentamplifier CA; two independent detector circuits respectively includingthe copper-oxide rectiflers DI and D2, having their inputs connected bytransformers T3 and T4, respectively, to the output of the ampliiier CA;a direct current amplifier AI having its input connected to the outputof the detector circuit including detector DI and its output circuitincluding plate battery (ground, since negative battery is used) and theresistance-condenser network I8 in series, connected via thetransmitting vario-losser `TVL across the mid-points of the secondarywinding of transformer T2 and the primary winding of transformer T3 tothe tap on the voltage divider G, H; and a direct current amplifier A2having its input connected to the output of the detector circuitincluding detector D2 and its output circuit including plate battery(ground), the resistance-condenser network I9 and the resistor 2l) inseries connected across the vertical diagonal of the receivingvario-losser RVL to the voltage divider Y, Z. Each direct currentamplifier AI and A2 may comprise a single amplifying vacuum tube havingthree or more electrodes, as indicated diagrammatically.

The vario-losser HVL in the input of the control circuit CL comprises anetwork consisting of two series copper-oxide rectiers I3 and I4,shunted by a linear resistor l5, in each side of that circuit, and alinear resistor I6 shunted across the circuit between the rectifiers.The vario-losser HVL is connected from the output of the direct currentamplier A2 by a direct connection 28 from the plate of the amplifiertube through the retardation coil L3 to the mid-point of the primarywinding of the input transformer T5, and the two halves of that winding,and from the two halves of the secondary winding of the inputtransformer T2 to the voltage divider G, H.

A network comprising a Varistor 30, condenser 3l and a linear resistor32 in parallel, is connected in common to the output circuit of thedetector DI and the input circuit of the direct current amplifier AI,and a similar network comprising the varistor 33, the condenser 34 andthe linear resistor 35 in parallel, is connected in common to the outputcircuit of the detector D2 and the input circuit of the direct currentamrer A2. Varistors 30 and 33 may each comprise a block ci a materialcomposed of silicon carbide crystals and an insulating binder (kaolin),commercially known as Thyrite, which is disclosed in the United StatesPatent 1,822,742, issued September 8, 1931, to K. C. McEachron.

The varistors J and K operating as asymmetric impedance devices, whichmay be copper-oxide rectiers or any other non-linear resistance elementsof the unidirectional type, and the capacitor C, in combination with thevaristor-capacitorresistor arrangements 3D, 3l, 32 and 33, 34 and 35 inthe input of the direct current ampliers AI and A2, respectively, areAutilized to provide 'suite able time constants for the control of thevariolossers TVL, RVL and HVL to accomplish the purposes of theinvention in a manner which will be described later in connection withthe complete decription of the operation of the circuit of Fig. 4. Thevaristor J and the capacitor C in series are connected across the inputof the direct current amplifier A2 on the output side of the detectorD2, the varistor J being poled in opposition to the detector D2; and thevaristorl K is connected between the upper terminal ci the secondarywinding of transformer T3 on the .input side of detector DI, and a pointbetween the varistor J and the capacitor C, and is polled in the samedirection as the varistor J. Also, the secondary winding of thetransformer T3 is transposed with respect to the secondary winding ofthe transformer T4 for a purpose which will be brought out in thefollowing complete description of the operation of the circuit of Fig.4. Other elements of the circuit of Fig. 4 will be described later inconnection with the following description of operation.

Normally, that is, when there is no speech input to the control circuitCL, the direct current amplifiers AI and A2 are biased beyond thecut-offV point and, since there is essentially no voltage drop acrossthe resistors in the resistance-condenser networks I8 and I9 resultingfromplate current flow, the plate of each of these amplifiers approachesground potential. It will be noted that the vario-losser pad RVL in thereceiving circuit RC is interposed between the plate of the directcurrent amplifier tube A2 and a potentiometer comprising the resistors Yand Z; that the vario-losser pad TVL in the input of the transmittingcircuit TC is interposed between the plate of the direct currentamplifier tube AI and a potentiometer comprising the resistors G and H;and that the vario-losser pad HVL in the input of the control circuit CLis interposed between the plate of the direct current amplifier tube A2and the potentiometer made up of resistors G and the potentiometerresistors Gand H are more negative than the potentials at the plateterminals of the direct current amplier tubes AI and A2, so thatnegative biasing voltages are applied to the rectiers in thevario-losser pads TVL, RVL and HVL causing each of them to exhibit ahigh resistance characteristic. Since the vario-losser pad RVL isconnected in shunt with the receiving circuit RC, the normal lossinserted in that circuit by the vario-losser pad is a minimum. On theother hand, the rectiilers in the vario-losser pads TVL and HVL are inseries with the transmitting circuit TC and the control circuit CL,respectively, so that these vario-losser pads normally provide a maximumloss in the circuits in which they are connected.:

Thus, any waves applied to the transmitting circuit TC or the controlcircuit CL when the operator is not talking will be highly attenuatedwhereas any waves including voice signals from` a distant party receivedover the line yconnectedl to the two-wire terminating section TS, willbe highly 'amplified in the receiving circuit RC.

When the operator talks into the transmitter cuit CL. -TheportionpassingV intwthe control circuit CL will be'attenuated inthevario-losser padHVL which atthe time provides a high lossl in'vtheinput of the control circuit CL.` The attenuated voice currents inthe output of the pad HVL will be impressed by the transformer T5 on theinput of the control amplier CA which will amplify them in accordancewith its gain setting.

A portion of the ampliiied voice signals in thev output of the controlamplifier CA will be impressed by the transformer T3 on the detectorcircuit including detector DI and the other portion will beimpressed bythe transformer T4 n ythe detector circuit includingthe detector'Dl. Therectified signal outputs of the` detectors DI and D2Will build up'potentials across the capacitors 3l and 34, respectively. Owingprimarily tothe presence of the resistor 24 in series with the detectorDI, the potential across the capacitor 3.I risesless rapidly than thatacross the capacitor 34. charged .throughthe varistor K by the signalsin the output of the transformer T3 to essentially the same potential asthe capacitor 34. The varistor J, because of its opposite poling,operates as a high resistor between the output of the detector D2 andthe capacitor C during the charging period and assures the independenceof the charging paths of the capacitors 34 and- C. This is required topermit a sufficiently rapid potential rise across the capacitor 34.Hence the chargingtime of capacitor 34l with the capacitor Cy in thecircuit is substantially the same as it would be if the capacitor C werenot present. The connection of the varistor K to the upper terminal ofthe secondary winding offtransformer l T3, instead of to the upperterminal of the secondary winding of the transformer T4, effectivelyeliminates the common resistance of the transformer winding so as toprovide maximum iso-V lation of thecharging circuits of thecapacitors-1i 34 andC. The transposition for the` secondary winding oftransformer T3 with respect to the secondary winding of transformer T4causesthe detector D2 land the varistor K to operate on opposingl halfcycles oi the alternating current wave supplied from the output of thecontrol amplifier CA and thus contributes to the faster operation(charging) of the circuit of the direct currentf amplier- A2- than thatof directccurrent ampliiierfAl.

The resistors 2.3, 2,2 and the resistors 21, 26 constitute voltagedividers between the battery B4 and ground, and between battery B5 andground,

respectively. The potential drops across resistors` 23` and 21 areappliedacross detectors DI and D2, respectively, as negative detectorbias. The potential drops across'resistors 22 and 26 provide negativegrid biases for the direct current ampli,-V ers A lv and A2,respectively. By proper selection of the resistance values for thepotential dividers,

thefbias applied to the direct current ampliiier tube'AI may be madesomewhat greater than that applied to the direct current amplifier tubeA2.y

The direct current potential developed across ca-Y pacitors 3| and 34 bythe rectied speech currents overcomesrthe negative biases, thus allowingplate current to'owthroughthe direct current. amplier tubes Al and A2,respectively. Since the rise of potential across capacitor 3l is slowerthan thatacross capacitor 34, and since the negative bias applied tothedirect current amplier tube AI issomewhat greater than that applied tothe direct current amplifier. tube A2,- plate. current.A

110W- through A2 is insured in advanceA of that through direct currentamplier tube Al. Asa

At vthe same time, the capacitor C will' be 8i result, the-potentialbetween the-plate'fof tube A2 and: ground caused. byplate` currentflowfthrough the resistor in the resistance-condenserfcircuitA I9'increases in advance of the corresponding increase in the potentialbetween the plate of tube Al and ground caused by plate current iiowthrough the resistor in the resistance-condenser .circuit I8 in theoutput of` tube AI. The circuit is so designed that, with plate currentow in theI direct current amplier tubes VAI and A2, the potentiallbetween ground and eitherplate-is greater than the voltage acrossresistor H or Z. Thus,` the. potentials across the rectifiers in thevariolossers TVL, RVL andl HVL are reversed 'in polarityA and reduce theimpedance of `theserectifiers.- Consequently, the series resistances 6and l5, in the vario-losser pads TVL andHVL, respectively, are shuntedby low resistance paths soA as to reduce the loss values Vof these padstotheir minimum values-determined by the value ofthe shunt resistors 7,8 and I6, respectively, of'theseipads; and the vario-losser RVL providesa very lowvimpedance shunt across the circuit RC Soas to provide maximumdesired loss in that circuit.

Under the above-mentioned vario-losser pad conditions, the loss inA thetransmitting circuit between the operators transmitter T'and thetransmitting amplifier TA is greatly reduced. This eii'ectivelyincreases the gainv in the voice transmitting branch TCfor the-operatorsvoice currents which are amplied by amplifier TA and then passed throughtransformer `TI and two-y Wire-section TS to the connected line,`while/cried tively reducingthe gain of the voice-receiving circuit RC sothat the transmission of sidetone energy through the latter circuit isnot increased. The input ofv the receiving amplifier RA'is essentiallyin parallel with the line and hence the highY loss in theV receivinglcircuit RC is required to maintain the sidetone level to the operator ata satisfactory value; The transmitting variolosser pad TVL largelydetermines the room noise sidetone llevel to the operator whereasthereceiving pad of vario-losser pad RVL largely determines speechsidetone level.

Owing to the time sequence inherent in the operation of the detectorcircuits DIv andDZ as described above, the-high loss is'inserted in'thereceiving circuit RC by the vario-losser'pad'RVL and the'high lossin theinput of the control circuit CLv is removed by the vario-losser pad HVL-before the high loss is removedin the trans'- mitting circuit TC by thetransmitting variolosser pad TVL. As explained in connection withthe-`curves of Figs.l 2 and 3, this sequence of operation is necessaryto prevent -atransient whichwould arise during the operating intervaloffthe controlfcircuit CL if the high loss conditionv of thetransmitting vario-losser pad TVL were removed before the high-.lossv isinserted in the receiving circuit RC by the receiving Vario-10sserpadRVL. Thelow loss condition of the vario-losser pad HVLinthe input ofthe control circuitCL pro.- duced by operation of the :latter circuit inthe manner described above, increases thev sensitivity of the controlcircuitV CL during'the ntervalthat the transmitter unit Tis actuated `bythe operators voice. This increased sensitivity insures the holding ofthe transmitting variolosser pad TVL and the receiving vario-losser padRVL in the off-normal condition over a wide'variation r in speech levelssome of which maybe of llower intensity than occasional room noisepeaks. In this way the HVL vario-losser permits operation under somewhathigher peak conditions of room noise without aiecting the ability ofimpressed speed to maintain the control circuit in its oft'- normalcondition.

When the operator ceases talking and the inputs to the detectors DI andD2 in the two detector control branches of the control circuit CL areremoved, the voltages built up across the capacitors 3l and St in theload cicuits of these detectors will be discharged through theassociated circuits. In the load circuit of the detector DI, thecapacitor 3l will discharge through the resistor 32 and the varistor 3U.As this discharge occurs, the potential between the grid and cathode ofthe direct current amplifier vacuum tube Al decreases accordingly toreduce the plate current through that tube. As the plate current throughtube Al decreases, the potential between its plate and ground will alsodecrease. When the capacitor 3l has discharged sufliciently, thepotential from the plate of tube AI to ground will become smaller thanthe voltage drop across the resistor H. At thismoment, the bias on thetransmitting vario-losser pad TVL becomes negative and increases theimpedance of that pad. The restoration time of the detector loadcircuits is determined primarily bythe values of resistance andcapacitance in these circuits. The characteristics of the siliconcarbide varistor 3B or 33 used are such that its resistance is a directfunction of the voltage across it. Hence, the varistor tends to maintainconstant detector characteristics in spite of voltage variations causedby relatively wide differences in sound level input. When no speed inputis present, the associated detectors may develop a small voltage frombackground noise inputs. If the gain through the control amplifier CA iscorrectly ad justed with respect to the detector and amplifier biases,this voltage will be insuiicient to alter the operation of the circuitas described above.

While the circuit of the detector Dl is restoring to normal, similaraction occurs in the circuit of detector D2 and its associated receivingvariolosser pad RV'L and control circuit vario-losser pad HVL. In thelatter case, the capacitor 34 discharges through the resistor 35 and thevaristor 33. However, as the potential across capacitor 34 decreases, anincremental positive bias is developed across varistor J owing to thecharge on the capacitor C. As a result, the capacitor C dischargesthrough the varistor J and in parallel with capacitor 34 throughresistor 35 and varistor 33. It should be noted that' the capacitor C isarranged to charge independently of capacitor 34, but to discharge inparallel with it so as to increase the time constant of the circuit ondischarge. By this arrangement, the voltage for causing operation of thedirect current amplifier Al under control of the detector DI is made tobuild up more rapidly than the voltage for causing the operation of thedirect current ampliiier A2 under control of the detector DI but todecay more slowly. As a result, the receiving variolosser pad RVL andthe control circuit variolosser pad HVL losses are switched prior to thetransmitting vario-losser pad loss during the operate interval but arerestored subsequently to the removal of the transmitting vario-losserpad loss during the restoration interval. The object of this padswitching sequence is to maintain the high loss of at least onevario-losser pad in the sidetone circuit at all times. If this sequenceis destroyed, a switching transient will occur during operation,restoration or both. When the receiving vario-losser padRVL and thecontrol cir cuit vario-losser pad HVL are restored to the ccnditions `inwhich they provide low loss and high loss, respectively, in the circuitsin which they are connected, vthe operators telephone set has returnedto normal. The addition of varistors J and K with capacitor C,therefore,` materially increases the discharge time constant withoutappreciably aiecting the charging time constan-t (operate) so as toproduce a fast operate, slow release condition for the vario-1ossercontrol circuit associated with detector DC.

The retardation coil L3 in the lead 28 connecting the plate of thedirectcurrent amplifier A2 to the mid-point of the primary winding oftransformer T5 is used to isolate the simplex control circuit of thevario-losser pad HVL from audio frequency components present in theplate circuit of the direct current amplifier tube A2. The retardationcoil L2 in the connection between the plate of the direct currentamplifier l tube Al and the mid-point of the primary winding oftransformer T3' performs a similar function between the transmittingvario-losser pad TVL and the plate circuit of the direct current Thearrangement VL comamplifier tube Ai. prising two oppositely polednon-linear resistors in parallel, connected across the receiver R isprovided to limit the maximum level output from the receiving circuitRC. The circuits NI in the input of the receiving circuit branch-RC areprovided to increase the high frei the ain-pliers in thetransmissionpaths instead l of separate loss devices therein to suitablycontrol the relative losses of these paths.

What is claimed is: l -l 1. In a signal transmission system, a two-wayline, two oppositely-directed one-way trans-- mission paths forrespectively 4transmitting signals to and receiving signals fromsaid-line,

means normally providing a high transmission loss in one of said paths,`means normally providing a low transmission lossl in the other ofsaidVpaths such as toV allow substantially free transmission of signalsthereover, switching means comprising two switching lbranches having acommon input circuit fed fromsaid one f path near its input, one of saidbranches being l operatively responsive to the signal energy aplpliedthereto through said common input cir'-` cuit when signals are impressedon the input of said one path to reduce the loss therein toa relativelylow value such as to allow substantially free transmission of theimpressed signals over that path, and releasing in response to cessationl in the impressed signals to restore said one path l to its normal highloss condition, the other of said branches being operatively responsiveto the signal energy applied thereto through said common input circuitwhen signals are impressed on the input of said one path to increase theI loss in said other pathto a relatively high value, and releasing inresponse to cessationin the i1nl pressed signals to restore said otherpath to its. normal low loss condition and means to reduce.,

transient` disturbances introducedv ,inrsaid paths when` signal-1transmission kisiy switched from.` one patlttogthe others'byytheoperationrori release. of said?. switching means comprising means'v formakingl .therelative operate :time constants,v of `tine* twdswitcliingbranches such that in' response. tofthe signals impressed kon xsaidonepath t said: other switching branch7 operates before said one switchingbranch and.: means" for making the relative:releasertime-constants of'said two switch; ing branches such th'atfin' response to cessation in=the signals impressedfon said one path lsaid one-'switching branchreleases before' said other' switchingbranch. l

2;A Aatelephonef set comprising line terminals,y arvoice -signaltransmitting` circuit and a voicev signal. receiving circuit veachincluding amplifyv ing." means; connected-fvr to," said line terminals;means-normally providing ali'igli^loss in said transmitting circuitand alow'loss `in saidreceiving circuit, such-aste condition said set'normally forv signal vreceiving only, a control device connected to saidtransmitting circuit in frontlof-'the--loss means therein, said lcontroldevice comprising'two control branches respectively responsive tooutgoing voice signals-apn plied* to 'f the input4 ofi saidtransmittingcircuit1v to'switchthenormal'higl'i-'loss out of thatfcircuit andialhighloss intosaidrecei-vingcircuit so as tof'condition` said'set for vsignaltransmitting only, and responsive -to'cessation inthe supply ofVoutgoing 'signals' to -said transmitting "circuit to re' turny thevlossfmeans in said'transmitting and receivingcircuits to theirv normalhigh' loss andf lowrlossconditions,` respectively, and means toreducer-transient'disturbances introduced in said circuits whileswitching-'said set'from the transmitting*tohtlfiereceiving'conditionlor from the receivingto the transmitting condition comprising i meanslfor making the control branch for switching'loss into said receivingcircuit relatively` fast'operating',compared-with the control`branch'ior'switching loss out of -said transmit#v ting'cireuit andmeansonmakingthe controlv branch for switching loss `into said receivingcir cuit slow releasing cornparednlitliA the control branchV forVswitching `loss loutof said transmitting circuit:

31 .Tl'etelephone setof'clairn'Z, inwhicliv each of `saidfcontrolbrancliesincludes 'similar capacitor v meansadaptedtobe"chargedinresponse tof the appliedv voiceV signals to control lthe associatedlosses; and to `be"discharged'in'response-to cessa tionin'the'appliedvoice signals, so as tof'control the time constants of they respective'Acontrol branches, andthe last'menti'oned means come prises auxiliarymeans -for materially increasing the discharge timeof thescapacitormeans in the r control branch for switching high lossinto said`receivingcircuit without appreciably affecting the charging-- time `oiythat capacitor; means.-

4; A-telephone set comprising line`A terminals, separatefvoice signaltransmittingfand vciee'sig-k nai receivingv circuits, each including'amplifying v means,r connected-"tofsaid line terminals-` Variableloss'umeans` inffront" of 'the' amplifying'. means in' eachofisaidcircuits; the variablelosslmeans'inA said' transmitting v circuit beingnormally adj usted toA introduce a l high loss #therein and Ithevariable loss-means in said receivin'gcircuit being vnormally adjustedtoA introduce. alvery'- small loss `therein,` a control deviceJ fed fromsaid' transmittingcircuit ata point `innfrontl of the variableflossmeans therein, said control devce being operativelyre'A sponsive-to theivoice signals :applied tl'i'eretororn` said" transmitting-circuititot-adjust th'efV variablev loss-'means therein sothatit'introduces iavery low loss to outgoing signals andthus-eiectively.

increases the gainof'thatci'rcuit; and'to adjustintroducediinisaid;circuits duringvthe periods ci 10 adjustment of the:yvariable loss means therein comprising vmeans associated withsaid'control def vice:v for controlling itsf timeiconstants so that itoperatesin responsef'toappli'edivoice signals `to adjust fthe:vari'able= loss means `iny said receiving l5 justs -tli'elvariable-'loss means in said transmitting circuit Eto they lowlossycondition-1; and "in 'response' to ycessation in fthe vfapplied" voiceA signals releases infv such i mannerY as'.y to restore the variable Yloss' 2c==l rneans-insaidl transmitting circuit to lits normal high loss`condition 1= before 'restoring'the Variable loss 'rneansin saidreceivingfcircuit toits normal.low-losscondition.

5.1 A telephoner'set vIcomprising-'lineterminals, a

voice signal transmitting circuit andaff-voice ysig- 3c transmittingvcircuit beingy normally; biased' iso Yas to .introduce ai high loss in`that v-circuit' an'd- ?the vario-losser inl ther receivingA circuiltfb'eingfnor-*- mallyI-'biased IsoJ as: tof introduce yza/very smally`loss therein; aifcontrol circuit 'lforth'e tw'o vario-lossers"lconnected .to said :transmitting circuit -`at -a-pointf in'liront .ofthe variov-lossertherein," said'lcontrol' circuit having two -`controlf.branch-es each includ'- ingra. detectorefor :detecting azdiiierent*portion otf the: l voice signals: divertedin'to said? control foirecuitwhen outgoing-i voice if signalsfarei applied to l thefinput offsaidtransmittingcircuit;` thefdetected signalr output of one:4 of :said'control; branches `beingr. utilized: to changeftheebias.; on'` thevari'o- I losserf in isaid itransmittingf: circuit lso :that it'finl 45serts. aiveryvlowly loss :in thatcircuitllto thefout going signalsY and'the:@detected-signalI output' ofi the i other of :said control 'brancheslbeirlrgf utilized to: change the 'i bias i ont: the varioslsserin saidf* receiving.`v circuit so that it Afinserts'iai-liighf floss rin that ycircuit,V the zreduction 1in the Idetected signal '1 outputsv r ofsaid.. one? andy said other` control branches:` with cessationtin"the'application' of voice-k signals to" thea, inrmtofi salditransmittingcircuit respectivelyrestoringftonormal the-'biases 55, onf: the`vario=lossersiy in said; transmittingv circuit f andv said/receiving:4circuiti to reinsert 'said' f high 1 lossinrthe former circuitan'd'fsaid-loWlss in-"thef latter circuit; and' means-to .reducetransient rdisturbances introducedfinton Seidl-transmitting and 60receiving circuits during. the periods in' which` the loss lvalues ofthe' variollossers'l'therein' are beingchanged.comprisingmeansfforimaklng.said other control branch`fastoperatingaand'fslow releasing relative to saidoneicon'trolrbranch.-t a

in" said transmitting: and` receiving! circuits inthe'variablefloss"meansf in lsad'rireceivin'g circuit'v circuit-l tof the high* loss,zcondition before' it-ad' 6i ThefteIephOne *set Ioffclaimf?5:,3in whichthe* means 'for"changingftheibmsesfonz the vario-'lossers meanscomprises two asymmetric impedance devices, another capacitor connectedin series with one of said asymmetric impedance devices across thecapacitor and shunting resistor in the detector load circuit of saidother control branch, means for separately charging said other capacitorthrough the other of said asymmetric impedance devices by the voicesignals applied to said one control branch, said one asymmetricimpedance device being so poled with respect to the detector in saidother control branch as to isolate the charging circuit of said othercapacitor from the charging circuit of the first capacitor in thedetector load circuit of said other control branch, the discharge ofsaid rst capacitor through the resistor in shunt therewith in responseto cessation in the signals applied to the input of said transmittingcircuit applying a positive bias to said one asymmetric impedance devicecausing said other capacitor to be discharged through said oneasymmetric impedance device to increase the discharge time constant ofsaid other control branch with respect to that of said one controlbranch Without appreciably affecting the charging time constant ofeither of the two branches.

7. A telephone set comprising line terminals, a voice signaltransmitting circuit and fa voice signal receiving ycircuit eachincluding signal amplifying means, connected to said line terminals,variable loss means in front of the `amplifying means in each of saidcircuits, a control device fed from the input of said transmittingcircuit, a third variable loss means in the input of said controldevice, the variable loss means in said transmitting circuit beingnormally adjusted to introduce a high loss therein and the variable lossmeans in the receiving circuit being normally adjusted to insert `a lowloss therein so that said set is normally conditioned for signalreceiving only, said third variable loss means in said control devicebeing normally adjusted to provide a loss therein such as to appreciablyreduce the sensitivity of said device, said control device 'beingresponsive to the portion of the youtgoing signais applied thereto fromthe input of said trans- -mitting circuit to adjust said variable lossmeans in said transmitting circuit so that it provides a low loss to theoutgoing signals therein, to adjust the variable loss means in saidreceiving circuit so that it provides a high sidetone loss therein, andto adjust the variable loss means in said con.. trol device to reducethe loss therein land thus increase the sensitivity of that device, saidcontrol device being responsive thereto from [cessation in the supply ofvoice signals to said transmitting circuit to restore the variable lossmeans in said transmitting and receiving circuits and in said controldevice to their normal loss conditions, and means to reduce transientdisturbances in said circuits `during the changing ,of the lossconditions of the variable loss means therein comprising auxiliary meansassociated with said control device for controlling its operation andrelease time constants so that during signal transmitting intervals theloss of the Variable loss means in said receiving circuit is increasedand the loss of the variable loss means in said control device isreduced before the loss fof the variable loss means in said transmittingcircuit is reduced, and at the end of each signal transmitting intervalthe high loss condition of the variable loss means in. said transmittingcircuit is restored before the low loss condition and high lossconditions of the variable loss means in said receiving circuit and insaid control device, respectively, are restored.

JAMES N. WALTER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,332,430 Berger Oct. 19, 1943

